Aaron Reupert currently is a PhD candidate at the Laboratory of Glass Science with
Prof. Dr. Wondraczek where the focus of his work is on the side emission of fibers.
He holds a German diploma in physics from the Friedrich Schiller University of Jena
and wrote his thesis and published research on ion beam induced stress formation and
relaxation in germanium.

This paper investigates the possibility of coupling energy from guided optical fiber modes
to free space radiation modes by means of laser written scattering centers in the core of an
optical fiber. Such side emitting optical fibers are used for diverse illumination purposes,
including functional illumination, for example, in endoscopic diagnostics or phototherapy.
One aspect of great relevance comprises coupling the light guided by the optical fiber to
radiation modes while avoiding the coupling to cladding modes. Furthermore, the resulting
angular radiation pattern might be of interest for the desired application, e.g., uniform
environmental illumination which implies Lambertian emission.

So far, little research
has been conducted on the actual angular distribution of side-emitted light and on the
efficient coupling to free space radiation. We argue that this is due to the lack of a
practical investigation method. Therefore, here, we introduce the technique of leakage
radiation microscopy - or Fourier microscopy - to probe the processes of light scattering
on microstructures. Exemplarily, we consider features which were created on optical fiber
by focused femtosecond (fs) laser beam irradiation for direct imaging of the angular
spectral distribution of scattered light.